Introduction

In the context of the Himalayan tectonic settings, the undulating geological conditions play a pivotal role in influencing ambient background radiation. This radiation can encompass varying concentrations of radon, ranging from modest to severe levels. This is particularly concerning due to the potential release of significant decay products, which can pose harm to human health upon inhalation.

Radon (222Rn) and thoron (220Rn) are generated through the alpha decay of radium (226Ra) in the uranium (238U) decay series and radium (224Ra) in the thorium (232Th) decay series, respectively [1]. The distinct half-lives of 222Rn (3.8 days) and 220Rn (55.6 s) contribute to the differential distribution of residential sources. Specifically, 222Rn primarily originates from the foundations and walls of rooms, while 220Rn predominantly emerges from walls [2]. As both radon types are emitted from the ground and walls, their decay products are released into the atmosphere. Upon inhalation, the decay products of 222Rn and 220Rn are deposited within the bronchial tree's airways, thus exposing the lungs [2, 3]. The entry of 222Rn into indoor air occurs through minute cracks, fissures, and poorly sealed wall joints, thereby becoming accessible for inhalation and ingestion by individuals [4,5,6,7]. The difference in temperature between outdoor and indoor spaces generates a pressure gradient, facilitating the entry of 222Rn into indoor air [6, 8]. Once inhaled, the densely ionizing alpha particles of.222Rn interact with lung tissues, causing DNA damage that is associated with the development of lung cancer [1, 9, 10]

Remarkably, radon (222Rn) stands as the primary contributor to public radiation exposure, accounting for more than half of the total radiation humans are exposed [2]. Elevated quantities of 222Rn and its progenies within indoor spaces can pose substantial health risks. The alpha particles emitted by inhaled 222Rn, particularly two of its progenies, polonium (214Po and 218Po), hold significant destructive potential for lung tissue and have been linked to the occurrence of lung cancer in humans [11]. The WHO [12] scaled222Rn as the second leading reason of lung cancer, among smokers, and as the leading cause among all non-smokers. Both 222Rn and 220Rn, along with their decay products, are widely acknowledged contributors to human natural background radiation exposure, accounting for more than 50% of the total. Additionally, the inhalation doses of 222Rn and 220Rn within indoor air are predominantly influenced by the concentrations of their decay products [3, 13, 14]. Indoor background radiation exposure and concentrations are impacted by factors such as 222Rn exhalation rates, ventilation conditions, and building materials [15,16,17,18,19].The results of the present study can aid in understanding the health concerns connected to radon and thorium exposure in related geological settings. The concentration data and related dose estimations can help in identifying potential dangers to health and formulating effective mitigation plans.

In past studies [10, 20,21,22,23], the contribution of 220Rn inhalation and its decay products was often overlooked due to its short half-life. Notably, an investigation of gamma dose rates across forty-six locations in the Uttarakhand Himalayas, India, under various geological stresses, revealed moderate variations in dose rates [24]. Nevertheless, the higher dosage conversion ratio associated with 220Rn decay products underscores its significance, particularly in scenarios characterized by elevated 220Rn levels, such as homes constructed using materials rich in thorium. This current study aims to assess the environmental background radiation exposure resulting from the decay products of both 222Rn and 220Rn, within different dwelling materials in the geological settings of the Garhwal Himalayas.

Methodology

The experimental site is in the Central Himalayan region situated along the Himalayan Frontal Thrust (HFT) and the Main Boundary Thrust (MBT) regions, the most tectonically active regions. The geophysical and tectonic consequences are linked with the emission of soil gas. This experimental site is focused on frequent monitoring and in-depth investigation in accordance with the health concerns of residents.

In the present study in the tectonically active zone in the Garhwal Himalaya, different types of houses made up of different building materials were used. Therefore, the current study aimed to estimate the radiation doses received by the inhabitants due to 222Rn, 220Rn, and their progenies. The RADUET CR-39 films are used to record the presence of alpha particles in different dwellings of the experimental site. The detectors were placed in the bedrooms and the living rooms of the study regions at the 2 m height from the surface [25]. After the exposure of 3-months, these CR-39 films underwent a chemical etching process in a 6.25 normality NaOH solution at a temperature of 90 °C for a duration of 6 h. Following the etching process, the CR-39 films were subjected to analysis within an automated track-counting system. This analysis aimed to determine the density of alpha particle tracks. Subsequently, the 222Rn concentrations were derived using different factors such as the density of background tracks, the duration of exposure, and specific calibration factors. These calibration factors were established through exposure to both222Rn and 220Rn utilizing specialized chambers located at the National Institute of Radiological Sciences, Japan. To ensure the quality and reliability of the detectors, inter-comparison was conducted. The minimum detection limit (MDL) used in the present study were 3 and 14 Bq m−3 for radon and thoron (if concentrations were supposed to be 40 and 100 Bq m−3) respectively and the MDL values depend on the obtained concentrations [26, 27].

The following formulae were used to calculate the indoor 222Rn and 220Rn concentrations [28, 29]:

$$ C_{T} = \left( {T_{Rn + Tn} - T_{Rn} } \right) / \left( {d \times k_{Tn} } \right) $$
(1)
$$ C_{R} = \left( {T_{Rn} } \right) / \left( {d \times k_{Rn} } \right) $$
(2)

where kRn and kTn stand for calibration factor for the 222Rn (0.0172 ± 0.002 tracks.cm−2/Bq.m−3d) and 220Rn (0.010 ± 0.001 tracks.cm−2/Bq.m−3d) respectively. CT and CR stand for the 220Rn and 222Rn concentrations (in Bq m−3), and TRn, TRn + Tn stand for the densities of the tracks in the “222Rn” and “222Rn + 220Rn” chambers, respectively.

The indoor progeny concentration of the radon and thoron progeny can be calculated by the below equation [30, 31]:

$$ {\text{EETC }} = T_{{{\text{DTPS}}}} / d \times S_{Tn} $$
(3)
$$ {\text{EERC}} = \left( { T_{{{\text{DRPS}}}} / d \times S_{Rn} } \right) {-} {\text{EETC}} $$
(4)

where EERC and EETC stand for the equilibrium equivalent due to the concentration of the radon and thoron (Bq m−3). TDTPS and TDRPS stand for the densities of the tracks, STn and SRn stand for sensitivity factors for DTPS (0.09 trackscm−2d−1/EERC (Bq m−3)) for DRPS (0.94 trackscm−2d−1/EETC (Bqm−3)) respectively. The average value of the thoron progeny concentration for the same month was also derived from their exposure time and material categories of dwellings. The 220Rn progeny (Bqm−3) concentration under different environmental settings was explored in the present study.

Equilibrium factors for thoron (220Rn)

The radon (222Rn) and thoron (220Rn) equilibrium factors can be calculated by the below equation [32,33,34,35]:

$$ F_{Rn} = {\text{EERC}}/C_{R} $$
(5)
$$ F_{Tn} = {\text{EETC}}/ C_{T} $$
(6)

where FTn and FRn stand for the equilibrium factors of thoron and radon. EERC, EETC, CR, and CT are in Bq m−3 as discussed above in Eqs. 3, 4, 1, and 2 respectively. In the present study, only the thoron equilibrium factor (FTn) was calculated due to not getting radon progenies concentrations.

Annual inhalation dose (D) and annual effective dose (AED) calculation

The annual inhalation dose (D) and annual effective dose (AED) of radon, thoron, and their progeny were estimated by the following equations [10, 21, 36, 37]:

$$ D = \left[ {\left( {0.17 + 9 \times F_{Rn} } \right)C_{R} + \left( {0.11 + 40 \times F_{Tn} } \right)C_{T} } \right] \times 0.8 \times 8760 \times 10^{ - 6} $$
(7)
$$ \left( {{\text{AED}}} \right)_{Rn} = \left[ {\left( {0.17 + 9 \times F_{Rn} } \right)C_{R} } \right] \times 0.8 \times 8760 \times 10^{ - 6} $$
(8)
$$ \left( {{\text{AED}}} \right)_{Tn} = \left[ {\left( {0.11 + 40 \times F_{Tn} } \right)C_{T} } \right] \times 0.8 \times 8760 \times 10^{ - 6} $$
(9)

where CR and CT are the annual indoor radon and thoron concentrations, respectively with an annual indoor occupancy factor of 0.8. (AED)Rn and (AED)Tn are the annual effective dose due to the concentrations of radon and thoron (mSv y−1) respectively. FRn and FTn are the equilibrium factors explained in Eqs. 5 and 6 respectively [20, 38, 39].

Results and discussion

The radon and thoron concentration in the present study was carried out in the different houses of the different building materials. The first three of the samples (serial number 1–3) was measured in the houses made up of wood and stone, four of the samples from the houses made up of cement only (serial number 4, 5, 7, 11), three of the samples from the houses made up of wood and mud (serial number 8–10), and one sample from the house made up of wood and cement (serial number 6). Radon concentrations in all three season was found to be higher than other samples in the houses that are made up of cement only and mixed up with cement. The thoron concentrations were found to be higher than other samples in the houses made up of mixed wood. (wood+ stone in January to March and July to September, and wood + mud in April to June).The obtained concentrations are comparable to the other tectonically active zones [40,41,42,43,44]. Table 1 demonstrated the average value of 222Rn and 220Rn concentration (Bq m−3) in different seasons during the sampling period with different exposure times. Annual average 222Rn and 220Rn concentrations have been estimated in different indoor environments and the statistical data of the study is presented here.

Table 1 The average value of 222Rn and 220Rn concentration (Bq m−3) in different months of the year with their exposure time
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The season-wise values of the equilibrium factors for thoron with its annual average values are presented in Table 2.

Table 2 The equilibrium factor for thoron in all three seasons and its annual average value
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The box plot for the annual average equilibrium factor is shown in Fig. 1, in which the maximum of the data points lies in between the third quarter of the box. All of the samples ranged below 0.020 except one sample (0.029). The mean value is near about 0.015 while the median is less than the mean value. All of the data points were inside the kernel density’s core region, hence representing the positive skewness. The statistics show that the data sets of the annual average equilibrium factor were not distributed widely in the study region.

Fig. 1
figure 1

The box and Whisker plot for the annual average equilibrium factor of thoron

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The observed values of annual inhalation (D in mSv y−1) and annual effective doses (AED) are given in Table 3. The annual inhalation dose due to radon only, thoron, and its progeny ranged from 0.83 to 2.99 mSv y−1(with an arithmetic meanof 1.94 mSv y−1). The mean value of AEDs due to thoron, and its progeny ranges from 0.58 to 2.78 mSv y−1 (with an arithmetic mean of 1.75 mSvy−1). Both of the calculated values are well below the International Commission on Radiological Protection’s (ICRP) recommended value of 3 to 10 mSv y−1[45, 46].

Table 3 Total annual inhalation dose due to radon only, 220Rn and its progeny,and annual effective dose due to 220Rn and its progeny (mSv y−1)
Full size table

Figure 2 represents the box and whisker plot for the annual inhalation dose due to radon only, thoron and its decay products (upper part), and the annual effective dose due to thoron and its progeny (lower part). In the upper part of the graph, approximately 80% of the samples were below the 2.5 mSv y−1 and in the lower part of the graph, approximately 75% of the samples were ranging below 2 mSv y−1. All of the samples in both the upper and lower parts (except one) of the graph were inside the kernel density’s core area. The median and mean values for both parts of the graphs were obtained in the third quartile and the median was more than the mean values.

Fig.2
figure 2

Box and Whisker plot for annual inhalation dose due to radon only, thoron and its decay product (upper part), and annual effective dose (lower part) due to thoron and its decay product

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Figure 3 shows the graphical representation (line and symbol plot) for the annual inhalation dose due to radon only, thoron and its decay product (black colour indication), and annual effective dose (red colour indication) due to thoron and its decay product. It can be seen from the Figure that a similar trend was followed by both the parameters in which the maximum value was obtained at approximately 3 mSvy−1and the minimum value was obtained between 0.5 to 1 mSv y−1.

Fig. 3
figure 3

Line and symbol plot for annual inhalation dose caused by radon only, thoron and its decay product (upper part), and annual effective dose (lower part) caused by thoron and its decay product

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The correlation coefficient for annual inhalation dose caused by radon only, thoron and its decay product (right side), and annual effective dose caused bythoron and its decay product (left side) within the annual average equilibrium factor (FTn)are shown in Fig. 4. The Pearson’s r value between annual effective dose caused byradon only, thoron and its decay product and annual average equilibrium factor (FTn) was found 0.65 (p values = 0.027) whereas between annual effective dose due to thoron and its decay product and annual average equilibrium factor (FTn) was found 0.63 (p values = 0.034). Both of the parameters showed a strong correlation with the annual average equilibrium factor. The p values showed that the correlation between the above parameters is significant.

Fig. 4
figure 4

Correlation coefficient for annual inhalation dose (right side) and annual effective dose (left side) within annual average equilibrium factor (FTn)

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The correlation coefficient ofthe annual inhalation dosecaused by radon only, thoron, and its decay product (right side), as well as the annual effective dose caused bythoron and its decay product, has been analyzed as shown in Fig. 5. It can be seen from the Figure that a strong positive correlation coefficient (Pearson’s r value = 0.99) was obtained between them which means that there is a linear relationship between these two parameters.

Fig. 5
figure 5

Correlation coefficient relationships among annual inhalation dose due to radon only, thoron and its progeny, and annual effective dose due to

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Conclusions

The current study indicates that both (radon and thoron) indoor concentrations and equilibrium equivalent radon and thoron concentrations change considerably with season. The indoor radon and thoron concentrations observed are higher than the World Health Organization (WHO) suggested guideline level of 100 Bq m−3 as a consequence of the inadequate ventilation and low humidity, the projected seasonal value of the equilibrium factor for thoron and its offspring (FTn) is highest in the winter and lowest in the summer. The average yearly radon, thoron, and their offspring inhalation exposure is much lower than the UNSCEAR-suggested reference level. The calculated annual radon effective dosage is below the WHO and ICRP reference limits in the specific geographical area of the Garhwal Himalaya region, these dose assessments offer useful baseline information on radon and thoron exposure levels.